New Yorkers were shocked to learn last month that the NYPD won’t be receiving any of the $1 billion in stimulus funds earmarked for police hiring nationwide. As its reason for bypassing New York City, the Justice Department suggested that our crime rate is so low, we don’t need any more cops.

Of course it was to keep crime down that the city requested funds to eventually hire 2,000 more officers and was expecting an initial outlay to support 200 to 300. Shortly after the turndown was announced, the Department of Homeland Security did find funds to permit the NYPD to assign 120 additional officers to transit patrol. But this was only a drop in the bucket.

Yes, New York has had huge crime reductions over the past two decades. Murders, for example, are down from the range of 2,200 a year in 1990 to around 400 this year. But it’s a mistake to simply think these low crime rates are inherently permanent — as foolish as believing, a couple of years ago, that the stock market (or home values) would always keep climbing. The bad old days could easily come back.

In the 1980s, drug dealers turned New York streets into shooting galleries, mowing down rival gangsters and innocent citizens alike. Finally, the city government, in partnership with the state, put forth a “Safe Streets” plan that provided funds to significantly increase the police force. Over the next 10 years, the number of NYPD officers rose from about 30,000 to 40,000. With these reinforcements, the department was able to implement a galaxy of new strategies — yielding today’s remarkably lower levels of crime. But it’s worrying that the force has been slowly shrinking for years, even while the city economy boomed for much of the last decade. And now, thanks to a new fiscal crisis, NYPD strength is projected to fall below 35,000 by year’s end. Because the NYPD has managed to keep crime going down anyway, some will argue that we shouldn’t worry about having fewer police. The answer is that there are looming problems that could overwhelm public safety in New York.

The state government, for example, recently made significant changes in the drug laws, ostensibly designed to provide treatment rather than prison for addicts. In fact, as Police Commissioner Ray Kelly and the city’s district attorneys point out, the new law will allow even drug dealers to escape jail by claiming to be addicts. Don’t be surprised if murderous drug gangs start shooting up our streets again.

And then there’s terrorism. New York City, already attacked twice and targeted several more times, always tops the list of threatened areas.

Of course, the NYPD since 9/11 has developed the most extensive and sophisticated police anti-terrorism program in the nation. In 1990, only 17 of its officers worked full-time on counterterrorism. Now 1,000 are assigned to prevent or respond to an attack and thousands more can be quickly mobilized to assist them.

But the “bad guys” have upgraded their operations, too. Among the possible scenarios that experts point to are: “dirty bombs,” designed to contaminate whole city districts; the release of chemical or biological agents in enclosed areas; a cyber-attack aimed at paralyzing our communications and power systems — and a Mumbai-style commando raid.

To deal with terrorist threats while keeping crime down, New York needs a new “Safe Streets”-type plan before things get bad. It should start with Washington agreeing to furnish a realistic amount of stimulus money — and the city holding the line on police strength.

Thomas A. Reppetto, the for mer president of the Citizens Crime Commission of New York City, is completing a book on se curity threats.